An Organometallic Super‐Gelator with Multiple‐Stimulus Responsive Properties

Abstract

Stimulus-responsive gels have recently attracted widespread attention as new functional materials for potential applications in sensors, [1] actuators, [2] shape memories, [3] drug delivery devices, [4] and displays. [5] One of the promising properties that organogels based on low molecular mass organic gelators (LMOGs) can offer is their reversible sol-gel phase transition as a result of external stimuli. [6] As far as we know, redoxresponsive organogels from LMOGs, however, are limited. Shinkai and coworkers [7a] reported the first example of organogels of this kind, which contains a redox-active Cu I / Cu II center. Besides, they also synthesized a series of quater-, quinque-, and sexithiophene derivatives bearing two cholesteryl moieties at the a-position. It was found that a sol-gel phase transition can be implemented by addition of oxidizing and reducing reagents.[7b] Zhu and colleagues [7c] prepared an electro-active LMOG containing a tetrathiafulvalene (TTF) entity. The gel formation can be tuned by means of oxidation/ reduction of the TTF group chemically or electrochemically. Although these gel systems are redox responsive, their properties, such as mechanical strength, flexibility, and sensitivity to external stimulus, are far from those required for practical uses. Therefore, creating instant, reversible, redox-responsive, and mechanically flexible organogels still remains a challenge. As a remarkable organometallic compound, ferrocene (Fc) contains an oxidizable metal ion, Fe II , and is a nonpolar compound in the neutral state, and thereby it dissolves readily in hydrocarbon solvents. This property, however, can be easily reversed by simple oxidation of the central ion. Our interest in stimulus-responsive supramolecular gel systems led us to consider the compound as a neutral-cation redox pair that may be employed to tune the gelling ability of a gelator containing it. Actually, the same idea has been adopted by a number of groups for studies of switchable complexation and molecular aggregation in micelles and vesicles. [8] However, all compounds containing the apolar ferrocenyl or charged ferrocenium moiety reported so far do not result in gelation, as documented for a number of solvents. [8b,8c] Introduction of metal ions is a practical way of giving organogels some smart properties.[9] For example, Sijbesma and coworkers [9a] designed and prepared two chloroform gels with reported a palladium-based organometallic LMOG that is able to catalyze C-C bond formation even in the gel state. We report here four novel cholesterol-appended ferrocene derivatives ( Fig. 1a; see Supporting Information for preparation details), and present first evidence for the gelation ability of organometallic compounds of this kind, and particularly the unusual redox-, mechanical-, and ultrasonic-controllable sol-gel phase transition phenomena. These gelators contain one redox-active ferrocenyl moiety and one cholesteryl residue linked by different diamino units. This design was chosen on the basis of the analys...

Gels with sense: supramolecular materials that respond to heat, light and sound

“…In a study of cholesterol-appended ferrocene gelator, Liu et al found that sonication of a transparent cyclohexane gel results in a viscous opaque suspension, whereas shaking or heating the material produces a transparent solution. 449 Komiya et al, meanwhile, observed that sonicating the platinum analogue of complex 37 for three seconds in cyclohexane affords weakly phosphorescent gels, which exhibit stronger emissions and more intense metal-ligand charge-transfer absorption bands after prolonged exposure to ultrasound. 222 Although NMR spectra indicate that complete aggregation occurs regardless of sonication time, the extended treatment appears to produce thicker bundles of fibres, in which increased ordering and rigidity of molecules diminishes the rate of non-radiative relaxation.…”

Tuning Soft Nanostructures in Self‐assembled Supramolecular Gels: From Morphology Control to Morphology‐Dependent Functions

“…[ 36,37,44,45 ] For the tubes, various kinds of nanotubes with different parameters or shapes could be acquired. For the sphere structures, derived structures such as nanofl owers or cottonlike structures often appear, [ 39 ] which can be extended to micrometer sizes in many cases. Sometimes micrometersized platelets [ 46 ] have been also encountered.…”

“…[23][24][25] Second is the morphological diversity. Besides the conventional nanofi ber structures, [26][27][28][29][30] many other structures such as nanotapes, nanobelts, nanotwists, [31][32][33] nanotubes [34][35][36][37] and nanospheres [38][39][40] could be obtained through gel formation. More interestingly, one kind of gelator molecule can sometimes form various morphologies, which could be controlled by the gelation environment.…”

Multistimuli‐Responsive Supramolecular Gels: Design Rationale, Recent Advances, and Perspectives

“…[30] Compound 6 is a typical electroactive gelator designed by Fang and co-workers. [31] They introduced Fc onto a cholesteryl group with a diamino unit as a linker, 6, and found that it could gel cyclohexane and CCl 4 with a CGC as low as 0.09 wt %. Compound 6 thus falls into the category of supergelators.…”